The invention pertains to the construction and manufacture of aircraft frame members of the type in which a fuselage section and a pair of associated wing spars are fabricated as a unit from lightweight, fiber reinforced matrix composites, and wherein the fuselage section and the wings spars incorporate sine wave webs for resisting shear loads, and multiple fiber chords are affixed along the opposed edges of the sine wave webs for resisting bending loads, especially bending loads applied to the wing spars.
The use of fiber reinforced resin matrix composites as the basic structural elements of aircraft bodies has well known advantages, including a highly desirable strength to weight ratio. In many cases the composites are fabricated to resemble the basic structural shapes of traditionally used metal parts, and are merely subtituted for the latter. On the other hand, those who have recognized the unique characteristics of composites, especially the manner in which they are fabricated, have proposed innovative composite structures which are impractical to fabricate from metal, but which yield aircraft frames that are superior, on an overall comparative basis, to conventional metal frames, and are capable of being manufactured at a lower cost.
One particularly promising composite structure incorporates a sine wave shear web in the wing spars and fuselage section of the frame. Wing spars made in the configuration of an I-beam having a sine wave web have a number of advantages over composite I-beams incorporating other web configurations, such as a planar web, a beaded web, a planar web having rib-like stiffeners affixed thereof, and a web made of a honeycomb structure. On a comparative basis, an I-beam wing spar with a sine wave web is second only to the honeycomb web in shear versus weight capability and manufacturability, and is far superior to the honeycomb web in repairability and volume displacement (important when the holow interiors of the wings are used as fuel tanks). In this latter regard, the wings of aircraft are often designed to form hollow fuel receiving compartments in which the stored fuel is in direct contact with the structural components of the wing. Experience has shown that a sine wave web, presumably because of the multiple sheet laminations which form the web, is far more resistant to contact with fuel than is a web formed by a honeycomb core sandwich which may absorb the fuel into the cells of the honeycomb and cause internal deterioration of the core if the web or moisture barrier on the web is damaged.
While the sine wave shear web thus has a number of desirable features, there has not previously been a satisfactory way of joining the sine wave web wing spars to a fuselage section that also incorporates a sine wave web in order to form a unitary frame. As defined herein, the term "frame" refers to a structural unit oriented in a plane transverse to the longitudinal axis of the aircraft and encompassing a central, fuselage section and a pair of outwardly and generally diametrically opposed wing spars. In the completed aircraft, a number of these frames are assembled in longitudinally spaced intervals to form the framework of the body with the fuselage sections forming the bulkheads of the fuselage and the wing spars forming the principal structural members of the wings. Lacking a satisfactory way of joining the wing spars to the fuselage section, it has been the most generally accepted practice to fabricate the fuselage section and wing spars separately and provide each with structure suitable for joining the parts with bolts and connective plates and brackets. However, bolted-on joints are prone to fatigue failure. Additionally, the high part count associated with such connecting hardware increases the initial assembly cost and requires a larger inventory of parts for maintenance and repair.
With this background, it is accordingly one object of the invention to provide a wing spar and fuselage frame incorporating sine wave webs as the principal structure for bearing shear loads and having a configuration and assemblage suitable for being fabricated as a unitary structure.
Another object of the invention is to provide a transition structure in a unitary aircraft frame for joining the roots of I-beam wing spars made with sine wave shear webs to a central fuselage section made with a circumferentially oriented sine wave shear web, in which the transition structure efficiently reacts bending induced tension and compression loads that develop in the wing spars where they join the fuselage section. A related object is to provide a transition structure for geometrically blending vertically oriented corrugations of the sine wave shear web in the wing spars with radially oriented corrugations of the sine wave shear web in the fuselage section in a manner that enhances the strength and durability of the wing spars to fuselage joints.
Still a further object is to provide an aircraft frame that is capable of being fabricated as a unitary structure from fiber reinforced resin matrix composites and which has the combined characteristics of substantial shear capability versus weight, minimum volume displacement, ease of repair, and ease of fabrication compared to other frame configurations made from composites.